Atomic mean dipole moment derivatives and GAPT charges

Mean dipole moment derivatives determined from gas-phase infrared fundament al intensity data for 30 molecules are compared with Generalized Atomic Pol ar Tensor (GAPT) charges calculated from wave functions obtained with 6-31G (d,p) and 6-311++G(3d,3p) basis sets at the Hartree-Fock, B3LYP density fun ctional, and MP2 electron correlation levels. With very few exceptions, the MP2 results are in better agreement with the experimental values than are the B3LYP results calculated with the same basis set, although the differen ces between these calculated results are often small. The Hartree-Fock resu lts deviate most from the experimental values. For all atoms studied here, C, H, F, Cl, N, O, and S, the MP2/6-311++G(3d,3p) results agree most closel y with the experimental values with rms errors of 0.059, 0.013, 0.044, 0.04 5, 0.030, 0.041, and 0.014E respectively. Although the calculated results f or charges between -0.5 and +0.5e seem to deviate randomly from the experim ental results, calculated charges ranging from +0.5 to +2.0e tend to be sli ghtly larger than the experimental values. This is a consequence of the fac t that the MP2/6-311++G(3d,3p) calculations tend to overestimate infrared i ntensity sums for molecules with more polar bonds and intensity sums above 500 km mol(-1). The results reported here show that the calculated charge v alues seem to be converging to the experimental values as the basis set bec omes more extensive, 6-31G(d,p) to 6-311++G(3d,3p), and as the electron cor relation level becomes more complex, Hartree-Fock to B3LYP density function al to MP3. Experimental mean dipole moment derivative values are shown to b e consistent with trends in atomic charge values expected from chemical arg uments for the halomethanes, hydrocarbons and Group IV hydrides.